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P.SREELATHATAM/14-24

SIGNAL RANSDUCTION & PERCEPTION OF AUXIN : CELL ELONGATION

Physiological effect of auxin : cell elongation

Auxin promote growth in stems and coleoptiles ,inhibiting growth in roots These are synthesized in shoot apex ad transported

basipitally to lower tissues. The outer tissues of dicot stems are the target of auxin

action. The minimum lag time for auxin – induced growth is ten

minutes. In avena coleoptile an soybean hypocotyle reach the

maximum growth rate after 30 – 60 minutes of treatments. This maximum represents five to ten folds increases over

the basal rate.

Auxin rapidly increases the extensaibility of the cell wall

Plant cell expands in three steps: 1) Osmotic uptake of water across the plasma membrane is

driven by the gradient in water potntial. 2) Turgor pressure builds up because of rigidity of the cell

wall. 3)Biochemical wall loosening occurs,allowing the cell to

expand in response to turgor pressure.

GR = m (Ψp - ƴ ) m = cell wall extensibility Ψp = turgor pressure Ƴ = yield threshold Auxins increase the cellwall extensibility.

Auxin induced proton extrusion acidifies the cellwall and increases the cell extension

According to acid growth hypothesis hydrogen ion act as a intermediate between the auxin and cellwall loosening

Source of hydrogen ion is the H+ -ATPase in the plasma membrane. Whose activity is increase in response to auxin.

The acid growth hypothesis allows five main predictions 1)acid buffers alone should promote short term growth

provided the cuticle has been abraded to allow the protons access to the cell wall.

2) Auxin should increase the rate of proton extrusion and kinetics of proton extrusion should closely match those of auxin induced growth

3) Nutral buffer buffer should inhibit the auxin induced growth

4) Compuonds that that promote proton extrusion should stimulate growth

5) Cell wall should contain wall loosening factor with an acidic ph optimum

Signal transduction pathways ABP1 Functions as an auxin receptor Calcium and intracellular pH possible signaling

intermediates Auxin increases the level of free calcium in the cell. Changes in cytoplasmic pH serves as a second messenger In plants auxin induces a decrease in cytosolic pH of about

0.2 times within 4 min of application Cytosolic pH is 7.4 ph optimum of plasma membrane H+

ATP ase is 6.5 decreases in cytosolic pH of 0.2 units could cause a marked increase in the activity of pasma membrane H+ATP ase

Decrease in cytosolic pH might also account for auxin induced increase in free intra cellular calcium by promoting the dissociation of bound forms

MAP kinase that play a role in signal transduction by phosphorylating protiens

Auxin appears to exert its effect on the cell cycle primarmarily by stimulating the synthesis of the major cyclin – dependent protien kinase (CDK):Cdc2(cell division cycle 2)

Auxin induced gene fall in to two classes :early and late One of the important functions of the signal transduction

pathways iniated when auxin bind to its receptor is the activation of a select group of transcription factors

The activated transcription factors enter the nucleus and promote the expression of specific genes

Genes whose expression is stimulated by the activation of pre existing transcription factors are called primary response genes or early genes

Early genes expression cannot be blocked by inhibitors of protein synthesis such as cyclo heximide

Expression of early genes is very short time ranging from few minutes to several hours

Primary response genes

1) Some of the early genes encode proteins that regulate the transcription of secondary response genes

Because that required for the long term response to the hormones

Because late genes requires de novo protein synthesis, their expression is blocked by protein synthesis inhibitors

2) other early genes involved in intercellular communication , or cell to cell signaling

3) another group of early genes is involved in adaptation to stress

Five major classes of early auxin responsive genes have been identified

Genes involved in auxin regulated growth and development

1)the Aux/IAA gene family 2)the SAUR gene family 3)the GH3 gene family stresess response genes : 4) Genes encoding glutathione s transferase 5) Genes encoding 1 amino cyclopropine 1carboxylic

acid synthase

AUX/IAA gene family: These genes encode short lived transcription factors that

function as repressors or activators of the expression of late auxin inducible genes

The expression of most of the AUX/IAA family of genesis stimulated by auxin within 5 to 60 min of hormone addition

SAUR gene family: Auxin stimulate the expression of SAUR genes within 5

to 10 min of treatment. Response is insensitive to cycloheximide. These are the convenient probe for lateral transport of

auxin during photo and gravitropism.

GH3 Gene family: These genes are found in soybean and arabidopsis These are stimulated by within 5 to 10 min Mutations in arabidopsis GH3 like genes resulted in

dwarfism and appear function in light regulated auxin responses

Because GH3 expression is a good reflection of the presence of endogenous auxin

Synthetic GH3 based receptor gene is DR5

Early genes for stress adoptations: Several genes encoding glutathione – s – transferase A class of proteins stimulated by various stress

conditions are induced by elevated auxin conc